A microporous polyolefin film has been widely used as a battery separator, a separation filter, micro-filtration membrane, and the like, due to chemical stability and excellent characteristics thereof. Among them, a secondary battery separator has high ion transport capacity through inner pores in addition to a function of spatial separation function between a cathode and an anode. Recently, in accordance with high capacitance and high output of the secondary battery, as one of a method for improving electric stability of the battery, a demand for improving the characteristics of the microporous film has increased. In the case of a lithium secondary battery, when thermal stability of the microporous polyolefin film is reduced, a short between electrodes may be generated together with damage or deformation of the microporous film due to temperature increase generated from abnormal behavior of the battery, and a risk of overheating, ignition, or explosion of the battery may be present.
Recently, under a condition requiring high output/high capacitance of the battery such as an information technology (IT), an electric drive vehicle (EDV), an electric power tool, an energy storage system (ESS), or the like, since ignition possibility and explosion possibility generated at the time of abnormal behavior of the battery may be several times to several ten times higher than those of the existing battery, thermal stability at a high temperature capable of handling the temperature increase of the battery has been urgently required.
In order to solve a thermal stability problem as described above, a method of forming a porous film using a polymer binder and inorganic particles giving heat-resistance property at least one surface of the microporous polyolefin film was used, and research into a technology of improving stability of the battery simultaneously with improving thermal shrinkage at high temperature has been conducted.
In addition, in the case in which the electric power tool or the vehicle using the high output/high capacitance battery, the electric power tool or the vehicle may be frequently exposed to severe vibration and impact while in use, and in a structure of the existing battery, shaking of an electrode plate may be generated by vibration and impact in a vertical direction. Therefore, friction between the electrode plate and the microporous film may be generated or the electrode plate may protrude out of the microporous film. In the case in which this phenomenon is accumulated or severe, an internal short may be generated to cause ignition and explosion. Further, the plate may be repeatedly shrunk and expanded whenever the battery is charged and discharged, but since degrees of shrinkage and expansion are different according to a coating state of an active material of each portion of the electrode plate and impregnation state of an electrolyte solution, the electrode plate may be warped in the case in which the battery is frequently charged and discharged or under over-charge or over-discharge environment. As a result, a possibility of generation of the internal short may be further increased due to the problem such as deviation of the electrode with respect to the microporous film, or the like. Therefore, stability and reliability of the battery may not be sufficiently secured only by simply improving the thermal shrinkage of the microporous polyolefin film at high temperature.
A porous composite separator including inorganic particles was disclosed in International Patent Laid-Open Publication No. WO 09/026,467 (Patent Document 1), but it may be difficult to uniformly impregnate and completely dry a solvent due to characteristics of a process of adhering the separator and an electrode coating layer using the solvent. In addition, in the case of adhesion by heat, since it may be difficult to uniformly adhere all of areas of the electrode and separator to each other, wrinkles may be easily generated in the battery. These problems cause deterioration of battery performance and productivity. A separator coated with a first adhesive polymer and a second functional polymer was disclosed in Korean Patent Laid-Open Publication No. 2004-0090935 (Patent Document 2), but it may be difficult to improve stability of the battery only by using the polymer having an adhesive property and functionality in abnormal behavior of the battery in which an internal temperature of the battery rapidly increases, such as overcharge, penetration, heat exposure except for vibration and impact.
As described above, according to the existing technology, it may be difficult to implement a separator capable of significantly improving stability of a battery against vibration and impact simultaneously with improving thermal shrinkage at high temperature.